1. Field of the Invention
The invention relates to electronic circuits. More particularly, the invention relates to wideband Voltage Controlled Oscillator (VCO) and resonant circuits used therein.
2. Description of the Related Art
Oscillators are used in many electronic devices to provide a frequency source. Oscillators are used in receivers and transmitters as frequency references or as local oscillators used to frequency convert signals. An oscillator uses a gain element, or amplifier, in conjunction with positive feedback to create a regenerative signal. The resulting oscillator has a frequency output where the gain is greater than or equal to, unity and the phase of the feedback signal is equal to zero. A frequency sensitive phase shifting circuit, or element, is often placed in the feedback path to create a particular frequency of oscillation. One such frequency sensitive phase shifting circuit is a resonant circuit, also referred to as a tank circuit.
A Voltage Controlled Oscillator (VCO) is a tunable oscillator. The output of the VCO is a signal at a frequency that may be characterized by the equation xcfx89(t)=xcfx890+Ku(t), where xcfx89(t) represents the angular frequency of the VCO, xcfx890 represents an initial frequency, or center frequency, of the VCO, K is the VCO gain, and u(t) is the control voltage signal. The resonant circuit largely determines the frequency of oscillation and a tuning sensitivity of the resonant circuit largely determines the VCO gain.
To create a VCO, the resonant circuit is comprised of at least one variable component. The reactance of the variable component is a function of a control signal, typically a voltage level, so that the frequency of zero phase, and consequently the frequency of oscillation, is also variable. If the VCO is required to tune over a large frequency range, the variable component must be capable of tuning the resonant circuit over the large frequency range. Possible circuit implementations for a variable resonant circuit capable of covering a large frequency range include a resonant circuit incorporating a highly sensitive variable component or a resonant circuit requiring an extended control voltage range. The first alternative presents some problems because the VCO gain (K), typically measured in terms of MHz/Volt for a radio frequency (RF) oscillator, becomes very high. A high VCO gain results in large frequency changes for relatively small control voltage changes and makes the VCO more susceptible to noise induced on the tuning input, or control line. The second alternative also has disadvantages because the required control voltage range is very large. Large control voltages may present a problem in battery powered electronics having limited supply voltage ranges.
Because the resonant circuit tunes the oscillator to the desired operating frequency, the quality factor (Q) of the resonant circuit is important in maintaining a specific output frequency at a given control voltage level. A lower circuit Q generates a more gentle phase response, whereas a higher circuit Q generates a sharper phase response. A higher circuit Q is desirable to minimize the effects of small phase variations on output frequency. For a given phase variation, the change in VCO output frequency is more pronounced in the circuit having the lower circuit Q. The magnitude of the frequency change in a low Q circuit for a given phase variation is greater than the magnitude of the frequency change in a high Q circuit for the same phase variation.
As noted above, a VCO using a resonant circuit to establish its output frequency may be used in a variety of communication devices, such as a receiver or transmitter incorporated into a wireless phone. The receiver or transmitter may be required to operate over multiple bands or an extended frequency range. The multiple frequency bands may be contiguous or may be disjoint. Furthermore, the receiver or transmitter may be battery powered and have a limited voltage range over which a resonant circuit may be tuned. Thus, what is needed is a VCO or a resonant circuit for a VCO that tunes over a wide range of frequencies, is insensitive to noise, and that requires a limited control voltage range.
A wideband VCO and method for generating a frequency signal tunable over a wideband are disclosed. The wideband VCO uses a resonant circuit tunable over a wide range of resonant frequencies. The resonant circuit includes voltage variable elements such that the resonant frequency, and thus the frequency of oscillation, may be electronically tuned. The voltage variable elements are arranged such that multiple control voltages determine the resonant frequency. A first control voltage is applied to a first set of tuning elements and operates as a coarse control of the resonant frequency. A second control voltage is applied to a second set of tuning elements and operates as a fine control of the resonant frequency.
The first control voltage determines a frequency band of operation and the second control signal determines the frequency of operation within the frequency band determined by the first control signal. The wideband VCO may be implemented within a frequency synthesizer or may be implemented as a frequency source within a wireless communication device, such as a wireless phone.